Because the tight junction divides the plasma membrane into two domains (i.e., apical and basolateral), epithelial cells are capable of vectorial transport, whereby an ion or molecule can be transported from one side of the epithelial sheet to the opposite side (
Fig. 2.11 ). The accomplishment of vectorial transport requires that specific membrane transport proteins be targeted to and remain in one or the other of the membrane domains. In the example shown in
Fig. 2.11 , the Na+ channel is present only in the apical •Fig. 2.10 Ciliaareapicalmembranespecializationsofsomeepithelialcells.Ciliaare5to10µminlengthandcontainarraysofmicrotubules,asdepictedinthesecross-sectiondiagrams.Left, Theprimaryciliumhasnineperipheralmicrotubulearrays.Itisnonmotileandservesasamechanoreceptor(e.g.,cellsoftherenalcollectingduct).Cellsthathaveaprimaryciliumhaveonlyasinglecilium.Right, Thesecondaryciliumhasacentralpairofmicrotubulesinadditiontothenineperipheralmicrotubulearrays.Alsointhesecondarycilium,themotorproteindyneinisassociatedwiththemicrotubulearraysandthereforeismotile.Asinglecellcanhavethousandsofsecondaryciliaonitsapicalsurface(e.g.,epithelialcellsoftherespiratorytract).(RedrawnfromRodat-DespoixL,DelmasP.Ciliaryfunctionsinthe nephron.Pflugers Archiv.2009;458:179.) membrane, whereas the Na+,K+-ATPase and the K+ channels are confined to the basolateral membrane. The operation of the Na+,K+-ATPase channel and the leakage of K+ out of the cell across the basolateral membrane sets up a large electrochemical gradient for Na+ to enter the cell across the apical membrane through the Na+ channel (intracellular [Na+] < extracellular [Na+], and Vm which is oriented with the cell’s interior electrically negative with respect to the cell’s exterior). The Na+ is then pumped out of the cell by the Na+,K+-ATPase, and vectorial transport from the apical side of the epithelium to the basolateral side of the epithelium occurs. Transport from the apical side to the basolateral side of an epithelium is termed either absorption or reabsorption: For example, the uptake of nutrients from the lumen of the gastrointestinal tract is termed absorption, whereas the transport of NaCl and water from the lumen of the renal nephrons is termed reabsorption. Transport from the basolateral side of the epithelium to the apical side is termed secretion.